I don't think the thing inside her is anything other than her own desires. I think the truth in this series has always been that while Utena does feel admiration towards the magical girls, her lust towards them has always been stronger, so trying to let admiration win and accept her defeat when she still has the chance to molest them would obviously end up like this. A team-up against her is entirely possible, but this series has always been very pro- Utena letting loose, so I'd be surprised if this new transformation is treated as being totally a bad thing, and I'd be SHOCKED if she ends up as less of a degenerate after this battle.